It'd be a simple way to add some variety to equipped mode matches if it were possible to get more options for the talisman types which could be connected to kit.

I can appreciate that when forgemaster was setup that it was possibly considered that some talismans types were considered more powerful than others, but even so you still see people using gear which has 6 creature binds connected to it, (so there must be some utility to what is presumably the least powerful talisman) - Would it really make that big of a difference if someone wanted to use 6 mana boosts, or 6 minor buffs at the exclusion of other talisman types?

I think it's a good thing that Heroic gear has to have an orange talisman while arcane gear has to have a green talisman and vigilant gets either in return for low deck size. That restriction characterizes the gear. Spells binded to a hand aren't the same as minor buff talismans which improve the value of creatures for free to start the game.

Atilor/rapidor/arntaos/offilor is 30%-40% attack buff for free. One of the drawbacks to binding spells is that you already have those spells in the deck or binding non-creature spells dilutes the creatures in a deck and vice versa. A too-large deck and too-small handsize means those binded spells may never get seen in a game. If you bind Magic Bolt, but you don't see a chance to use it until mid-game then then if you draw it in an opening hand it is using a spot in your hand that could have been useful in early game. There are drawbacks to binding spells whereas minor passives are guaranteed; which is fine if there are max 2 of them, but game-changing if there is not a restriction.

Multiple Major Buff talismans would conflict with the value of Megamorus, etc.

In theory there is room for a fourth gear set having Transmutation talisman as required. That one could double up on transmutation talismans. Transmutation is an awkward combo of transformation and alignment manipulation and I suppose it's lack of unified character is why there isn't a gear set modeled on it.

Isn't the opposite true though, those buffs only have utility if you have a particular spell type out on the map - If you're choosing more buffs to use, then you've less control over the specific creatures available to make use of those buffs.

Or I guess, you're spreading your buffs thinly. 6 buffs to Law/Neutral/Chaos would let you buff every creature on the map, but you still don't get as big of a boost than if you're only boosting Air creatures, with an Air staff, and 4 Air creatures bound.